A normal map is a specialized texture (typically a purple-blue image) that simulates fine geometric detail on a 3D surface without actually adding geometry. Each pixel of the normal map encodes a surface-normal direction — which way that point on the surface is 'facing'. The renderer uses those encoded directions when computing how light reflects off the surface, so it looks bumpy and detailed even though the underlying mesh is flat.
Normal maps are how nearly every real-time renderer (game engines, ArchiCAD visualization, AR/VR) gets brick walls, fabric weave, wood grain, leather texture, and stone roughness to look believable without requiring millions of polygons of actual geometry. They're cheap to compute (just a texture lookup at each pixel) and they survive close inspection in most situations.
AI rendering operates without explicit normal maps — the model has learned what bumpy materials look like as a complete visual pattern, not as geometry + lighting. The practical implication: you don't need to think about normal maps in AI prompts, but specifying material texture clearly ('reclaimed brick with visible mortar joints', 'oak with visible grain') is the equivalent direction that pushes the AI to render the surface with believable fine-scale detail.
See also